Heretofore, are known cutting tools, wear-resistant tools and the like employing as their substrate a cemented carbide or ceramics such as silicon nitride or silicon carbide and having their substrates coated with a diamond film in order to improve the wear resistance and adhesion resistance. Generally, such a diamond coating is formed by a chemical vapor deposition or a like process, and grains constituting the diamond film have a size larger than about 4 micrometers (μ). How the crystal grows in a coating process of such a diamond film of the prior art is shown in FIG. 3. According to the prior art method, when a substrate 5 is set in a CVD equipment, for example, and placed under predetermined conditions, diamond nuclei 1 will be produced on the surface of a substrate 5, as shown in FIG. 3(a). Then if the conditions setting is changed to grow the nuclei 1, they will develop principally in a direction perpendicular to the surface of the substrate 5 to grow into grains 2, as shown gn FIG. 3(b), and these grown grains 2 will join together so as to form a diamond film 6.
If the diamond has a larger grain size W as illustrated above, adjacent grains of the diamond film 6 will form generally sharp V-shaped recesses on the order of micron in the surface so that the film 6 will have unevenness in its surface to make the surface lusterless. Further, when using the diamond coating for cutting tools and the like, the above-mentioned unevenness just represent the surface roughness of such tools, and the unevenness will partially be transferred to a work to further the roughness of the work surface. Besides, since the unevenness capture and hold chips firmly, such a tool has problematically tended to cause adhesion during working with a resultant loss in performance.
To solve such problems as described above, diamond coated tools having a diamond coating or film comprising fine grains of about 1 micrometer or below in grain size are proposed recently. As a typical example, a patent document 1 (Japanese Provisional Patent Publication JP A H11-58106, pp. 3-5, and FIGS. 5, 6 and 7) discloses a diamond coated tool in which the diamond coating has a 3 micrometers or smaller grain size and a surface roughness of 3 micrometers or less in Rmax subject to JIS B 0601.
Another patent document 2 (Japanese Provisional Patent Publication JP A 2002-79406, pp. 2, 4-7) discloses a diamond coated tool in which the diamond coating has a 2 micrometers or smaller grain size. The invention disclosed in this patent document 2 provides also a method of manufacturing such a diamond coated tool permitting its diamond coating to have a grain size of 2 micrometers or below. How the diamond grows in this method of the prior art is shown in FIG. 4. According to this method of manufacturing a diamond coated tool of the prior art, diamond nuclei 1 is first produced on the surface of a substrate 5 as shown in FIG. 4(a), and the nuclei 1 are grown until the grain size W in the growth directions reaches approximately 1 micrometer as shown in FIG. 4(b), whereupon the growth is stopped. Then, as shown in FIG. 4(c), conditions are set again to produce nuclei 1 so that they emerge on the thus grown diamond grains 2. And these new nuclei 1 are grown to diamond grains 2 on the former diamond grains 2, as shown in FIG. 4(d). In this case also, the growth is stopped when the grain size W reaches 1 micrometer. Furthermore, this method of the prior art also proposes to form a diamond film 6 by repeating the process steps shown in FIGS. 4(c) and 4(d). According to the prior art inventions, it is said that the smaller diamond grain size is effective for reducing the unevenness in the surface of the diamond coating 6 to improve work surfaces in roughness.